How to load/import a PNG image as plane via Blender's Python API?
The use_shadeless has been replaced with shader which accepts values of DIFFUSE, SHADELESS or EMISSION.

bpy.ops.import_image.to_plane(shader='SHADELESS', files=[{'name':file_path}])


For other options, if you manually do an import image as planes and have python tooltips enabled you can find the property name in the tooltips